1. Field of the Invention
The present invention relates to a method for manufacturing a piezoelectric material such as a piezoelectric ceramic, and more particularly relates to a method for manufacturing a piezoelectric material having a perovskite crystal structure using a hydrothermal processing. The present invention also relates to method for manufacturing a piezoelectric element that utilizes this piezoelectric material, and more particularly a piezoelectric element that functions as an ink discharge drive source for an on-demand type of ink jet recording head.
2. Description of the Related Art
Most piezoelectric bodies which can be expressed by the formula ABO3 have a perovskite crystal structure. Of these piezoelectric bodies, those having a specific composition exhibit an electromechanical conversion action, and are utilized as piezoelectric elements. For instance, lead-zirconate titanate (abbreviated as PZT), in which lead (Pb) is used for A and a mixture of zirconium (Zr) and titanium (Ti) is used for B, exhibits extremely good piezoelectric characteristics, and has become an important piezoelectric material in actuators such as ink jet recording heads.
A variety of methods have been employed in the past to manufacture a piezoelectric material. A method in which oxides are mixed and sintered is known as a first example thereof. This involves mixing lead oxide (PbO), zirconium oxide (ZrO2), titanium dioxide (TiO2), and other such oxides, pre-firing the mixture, grinding this product, and then sintering the resulting powder. This method makes possible the molding of bulk materials in particular.
A sol-gel method is known as a second example. This involves coating a substrate with a metal alkoxide solution (sol) containing lead (pb), zirconium (Zr), titanium (Ti), or the like by spin-coating or another such process, gelling this coating, and sintering the amorphous thin film thus obtained.
A third example is sputtering. This involves performing sputtering using an alloy of lead (Pb), zirconium (Zr), titanium (Ti), or the like as the target, and heat treating the amorphous thin film thus obtained.
With the above methods, the composition of the metal elements which make up the piezoelectric material that is the final product can be controlled to a certain extent by determining the composition of the metal elements included in the raw material.
As another example of a method for manufacturing a piezoelectric material, the thesis xe2x80x9cApplication of Hydrothermal Mechanism for Tailor-making perovskite Titanate Films,xe2x80x9d IEEE proc. of the 9th Int""l Symp. on Electrets, Shanghai, China, Sept. 25-30, pp. 617-622 (1996), Wuping Xu, Masanori Okuyama, et al., discloses a technique of crystallizing a piezoelectric film precursor by a hydrothermal method. This hydrothermal method involves coating a substrate with a sol of an organometal in an amorphous state, then subjecting it to a hydrothermal processing in an alkali aqueous solution, thereby promoting the orientation of metal atoms and crystallizing them into a perovskite crystal structure.
This hydrothermal method has numerous advantages because a piezoelectric film precursor can be crystallized at a lower temperature than when a film is formed by any of the first to third methods mentioned above. For example, because a low-temperature process allows for a reduction in the internal stress generated in the film in the course of the crystallization of the piezoelectric film precursor, cracking can be prevented and it is possible to form a thick film of a piezoelectric material. The ability to form a piezoelectric thick film enhances the reliability of the film because of the weak electric field produced in the film when a voltage is applied. A low-temperature process also makes a line printer feasible because of the low internal stress in the film.
Incidentally, it is mentioned in the above thesis that an alkali aqueous solution is used as the treatment solution in the crystallization of the piezoelectric film precursor. However, since the pressure chamber substrate of an ink jet recording head, for example, is produced by the fine working of a silicon substrate, it is undesirable to use a strongly alkaline aqueous solution of potassium hydroxide or the like as the treatment solution because the substrate or the piezoelectric material ends up being etched. In view of this, from the standpoints of making the substrate and other components resistant to etching, promoting the crystallization of the piezoelectric film precursor, and so forth, a barium hydroxide aqueous solution or a lead hydroxide aqueous solution has been used as the treatment solution.
The inventor, however, discovered that the lead content in the piezoelectric material decreases if a piezoelectric film precursor such as lead zirconate-titanate (Pb(Zr,Ti)O3; PZT) is crystallized using barium hydroxide as the treatment solution.
FIG. 5 is the EDX (Energy Dispersive X-ray Analysis) spectrum of a piezoelectric film (Pb(Zr0.56Ti0.44)O3) crystallized by a hydrothermal processing with barium hydroxide alone, and FIG. 6 is the EDX spectrum of a piezoelectric film (Pb(Zr0.56Ti0.44)O3) formed by sol-gel method. In these figures, the horizontal axis is the characteristic X-ray energy, and the vertical axis is the X-ray intensity. It can be seen from these experimental results that the lead content in a piezoelectric film crystallized by hydrothermal processing with barium hydroxide is smaller than the lead content in a piezoelectric film formed by sol-gel method.
When the lead content decreases in a piezoelectric film, there is a drop in both dielectric and piezoelectric properties. Accordingly, if a piezoelectric element featuring a piezoelectric material crystallized by hydrothermal processing using barium hydroxide is used as an ink discharge drive source, the ink discharge characteristics may not be as good as in the past. Similarly, when a crystalline film of a piezoelectric material is manufactured by hydrothermal processing, a drawback is that the metal elements contained in the initial piezoelectric material are lost, and the composition of the manufactured product cannot be controlled as expected. On the other hand, SEM (Scanning Electron Microscopy) observation revealed that crystallization is hindered when a lead hydroxide aqueous solution is used as the treatment solution.
With a conventional crystallization method involving sintering, a step of sintering under high-temperature conditions is essential, which means that metal components such as lead (Pb) evaporate and scatter during heating. This causes a problem in that the component ratio of the intended piezoelectric material tends to deviate from the component ratio of the piezoelectric material actually obtained, and also requires finding some way to keep the vaporized metal components (such as lead) from being discharged into the outside environment, which drives up the cost.
In view of this, it is an object of the present invention to provide a manufacturing method with which a decrease in piezoelectric characteristics can be prevented and the proper lead content can be ensured in a piezoelectric film when the piezoelectric material is manufactured by a hydrothermal method. The present invention further provides a method for easily controlling the component ratio of the metal elements that make up the piezoelectric material. Also provided is a method for manufacturing a piezoelectric material that imposes less burden in terms of environmental concerns, such as preventing the outflow of metals.
It is a further object to provide a method for manufacturing a piezoelectric element that is an ink discharge drive source by the above manufacturing method, and for manufacturing an ink jet recording head with superior ink discharge characteristics. Another object is to provide an ink jet printer equipped with this ink jet recording head.
The inventor investigated the compositional changes of a hydrothermal processing in light of the above drawbacks, whereupon they found that the cause of these drawbacks is that a divalent metal element (A site element) is replaced with a metal element in the aqueous solution in the course of the hydrothermal processing. The inventor then hit upon utilizing this phenomenon to control the composition of the piezoelectric material that is the final product.
The present invention is a method for manufacturing a piezoelectric material having a perovskite crystal structure expressed by the formula ABO3 in which e symbol A represents at least an element xe2x80x9caxe2x80x9d comprising a first step of producing an oxide containing an element xe2x80x9caxe2x80x9d, and a second step of producing a piezoelectric material by subjecting the oxide containing the element xe2x80x9caxe2x80x9d produced in the first step to a hydrothermal process using an aqueous solution containing the element xe2x80x9caxe2x80x9d, wherein the amount of element xe2x80x9caxe2x80x9d contained in the piezoelectric material produced in the second step is increased due to its substitution for element xe2x80x9caxe2x80x9d contained in the oxide produced in the first step.
With this manufacturing method, it is possible to control the element (A site element) in a piezoelectric material expressed by the formula ABO3 and having a perovskite crystal structure. Also, a piezoelectric material can be manufactured in an environment with a lower temperature than when one is formed by sintering, and it is possible to manufacture the piezoelectric material in the form of a thick film, in larger size and in bulk.
The element xe2x80x9caxe2x80x9d used in the second step may be an element not contained in the oxide produced in the first step. This allows the element to be contained at the A site to be contained at the A site of the final product even though it is not contained in the oxide prior to the execution of the hydrothermal processing in the second step.
The oxide produced in the first step may be a piezoelectric material expressed by the formula ABO3 in which the symbol A represents at least an element xe2x80x9caxe2x80x9d, and having a perovskite crystal structure. This allows the hydrothermal process of the second step to be performed for the piezoelectric material that has already been crystallized in a perovskite crystal structure, and allows the element xe2x80x9caxe2x80x9d at the A site of this piezoelectric material to be substituted with the element xe2x80x9caxe2x80x9d.
The first step may involve subjecting an oxide in an amorphous state to a hydrothermal processing using an aqueous solution containing the element xe2x80x9caxe2x80x2xe2x80x9d, and crystallizing the oxide in the amorphous state. Thus, the hydrothermal processing may be carried out in a total of two stages in the first and second steps.
The method of the present invention for manufacturing a piezoelectric material may be such that the oxide produced in the first step is an oxide in an amorphous state, and the hydrothermal processing in the second step crystallizes the oxide produced in the first step. Thus, a treatment in which an oxide in an amorphous state is crystallized and a treatment in which the desired element is introduced at the A site of the piezoelectric material that is the final product may be carried out at the same time by means of the hydrothermal processing in the second step.
The hydrothermal processing in the second step is preferably conducted using an aqueous solution containing both the element xe2x80x9caxe2x80x9d and the element xe2x80x9caxe2x80x2xe2x80x9d or another element expressed by A. As a result, when the element xe2x80x9caxe2x80x9d is introduced into the final product, for example, an element that enhances the piezoelectric characteristics (such as lead) is used, and when the element xe2x80x9caxe2x80x2xe2x80x9d is made into an alkali aqueous solution used in the hydrothermal processing, an element that promotes crystallization (such as barium, strontium, or calcium) is used, allowing the crystallization of the oxide and the enhancement of the piezoelectric characteristics to be fully realized.
The ratio in which the element xe2x80x9caxe2x80x9d and the element xe2x80x9caxe2x80x2xe2x80x9d or another element expressed by A are present in the aqueous solution is preferably adjusted to between 2:8 and 4:6. Adjusting to this range allows the compositional ratio of the A site to be controlled in the piezoelectric material that is the final product.
The aqueous solution containing the element xe2x80x9caxe2x80x9d is preferably an alkali aqueous solution of a compound expressed by the formula a(OH)n (n=1, 2, or 3). Also, the aqueous solution containing the element xe2x80x9caxe2x80x2xe2x80x9d is preferably an alkali aqueous solution of a compound expressed by the formula axe2x80x2 (OH)n (n=1, 2, or 3).
The element xe2x80x9caxe2x80x9d is preferably lead. This allows the piezoelectric characteristics of the piezoelectric material that is the final product to be enhanced. Also, the element xe2x80x9caxe2x80x2xe2x80x9d is preferably one element selected from the group consisting of barium, strontium, lanthanum, and calcium. This allows the crystallization of the oxide in an amorphous state to be sufficiently promoted. It is also preferable for the element expressed by B to be zirconium and/or titanium.
The concentration of the aqueous solution used in the hydrothermal processing in the second step is preferably between 0.05 M (mol/L) and 2.0 M (mol/L). Also, the treatment temperature in the hydrothermal processing in the second step is preferably between 120xc2x0 C. and 200xc2x0 C. Furthermore, the treatment pressure in the hydrothermal processing in the second step is preferably between 2 atmospheres and 20 atmospheres. It is also preferable if the treatment time in the hydrothermal processing in the second step is between 15 minutes and 120 minutes.
The oxide in an amorphous state may be produced by coating and pyrolyzing with a sol containing an organometal.
The piezoelectric material may be composed of lead titanate (PbTiO3), lead zirconate-lead-lanthanum zirconate-titanate ((Pb,La)(Zr,Ti)O3), or lead zirconate-titanate magnesium-niobate (Pb(Zr,Ti)(Mg,Nb)O3).
The present invention is also a method for manufacturing a piezoelectric element, comprising the steps of forming a lower electrode, forming over the lower electrode a film of a piezoelectric material exhibiting piezoelectricity by the above-mentioned method for manufacturing a piezoelectric material, and forming an upper electrode over the piezoelectric material.
The present invention is also a method for manufacturing an ink jet recording head, comprising the steps of forming a diaphragm film over a substrate, manufacturing a piezoelectric element over the diaphragm film by the above-mentioned method for manufacturing a piezoelectric material, and working the substrate and forming a pressurization chamber at a site capable of transmitting displacement of the diaphragm film produced by driving of the piezoelectric element.
In another method of the present invention for manufacturing a piezoelectric material, the element xe2x80x9caxe2x80x9d is barium, the element xe2x80x9caxe2x80x2xe2x80x9d is lead, and the element expressed by the symbol B is titanium. Furthermore, the oxide produced in the first step is preferably lead titanate (PbTiO3) composed of acicular crystals. It is also preferable if the first step produces the oxide by Metal-Organic Deposition (hereinafter xe2x80x9cMODxe2x80x9d). This makes it possible to manufacture a piezoelectric material that is BaTiO3 composed of acicular crystals with a large grain size, and more accurately, barium-lead titanate expressed by the chemical formula (BaxPb1-x)TiO3, where x in the formula is thin the range of 0 less than xc3x97 less than 0.05, which was difficult to manufacture with a conventional method.
The piezoelectric material of the present invention is expressed by the chemical formula (Ba, Pb)TiO3, wherein the piezoelectric material is composed of acicular crystals, at a specific spacing there are dislocation layers in which lattice defects are present, and the spacing between adjacent dislocation layers is at least 10 nm. This makes it possible to provide barium-lead titanate with excellent voltage resistance and good piezoelectric characteristics. In particular, it is possible to provide barium-lead titanate expressed by the chemical formula (BaxPb1-x)TiO3, where x in this formula is within the range of 0 less than xc3x97 less than 0.05.
The piezoelectric element of the present invention comprises the above-mentioned piezoelectric material and electrodes with which voltage can be applied to this piezoelectric material. This makes it possible to provide a piezoelectric element that has excellent voltage resistance and exhibits a favorable amount of displacement (deformation) with respect to the application of voltage. The ink jet recording head of the present invention is characterized in that the above-mentioned piezoelectric element is provided as a piezoelectric actuator. The printer of the present invention is characterized by being equipped with the above-mentioned ink jet recording head as a printing means.